The present invention relates to a method of operating multistage hydraulic machinery. More particularly, the invention pertains to a method of operating a multistage hydraulic machine with movable guide vanes, which effects depression of the water level when starting a pumping operation or effecting a condenser operation.
To effect starting or a condenser operation of a large-capacity reversible pump-turbine or the like, it is general practice to employ a motor-generator or a starting motor which is directly connected to the main shaft. In such a case, it is common to conduct starting in air for the purpose of reducing the starting torque required for the motor.
In order to realize starting in air, it is necessary to expel water from the runner chamber. To effect expelling of water from the runner chamber, it is general practice to depress the water level in the state wherein the guide vane in the maximum-pressure stage is totally closed, while the respective guide vanes in the remaining stages are opened.
In a conventional multistage hydraulic machine with movable guide vanes, for example, a two-stage reversible pump-turbine, a water level depressing operation is conducted as follows. The guide vane in the upper stage is totally closed, while the guide vane in the lower stage is totally opened, and compressed air is forcedly supplied from the back-pressure chamber of the runner in the upper stage. The water present around the runner is passed through a return blade passage by means of the compressed air and is successively forced into the runner chamber through the guide vane in the lower stage, which is the final stage, whereby the water level is depressed.
Thus, the level of water around the pump runner is depressed by means of the compressed air so that the runners are exposed to the air when effecting starting in air.
However, if it is taken into consideration that the depression of the water level is originally effected for the purpose of reducing the torque required for starting the runners, it may not be necessary to expel the whole of the water present within the return blade passage and it may be sufficient to expel the water around the runner in each of the stages.
On the basis of this idea, a multistage hydraulic machine has been invented which is provided with a bypass tube which extends from a return blade passage portion such as to communicate with a draft tube, such as the shown in Japanese Patent Publication No. 38336/1972.
The above-described invention, however, involves the following disadvantages: (1) it is necessary to employ a by-pass tube having a considerably large diameter and to provide an automatic valve on the by-pass tube; (2) the return blade passage portion is originally narrow and, therefore, it is not easy from the viewpoint of space to provide the by-pass tube and the automatic valve at the return passage portion; and (3) if the by-pass valve should have a failure (for example, if the valve should open during a normal turbine or pump operation), leakage of water will produce a loss in the turbine or pump efficiency, and further, since the pressure in the return blade passage portion is high in a normal turbine or pump operation unlike that in the water level depressing operation, the by-pass valve may involve vibrations.
In the case of a reversible pump-turbine, it is necessary for it to be able to maintain the water level in a depressed state while the pump-turbine is rotating in the pumping operation mode. In view of this, the following problem may be experienced. In the pumping operation mode, the runner in one stage acts during the water level depressing operation such as to centrifugally blow away the cooling water supplied to the runner and the water dropping from the upper-stage runner chamber and to convey the water back to the higher-pressure stage. Since it is necessary for the above-described water to be finally discharged to the draft tube, the above-described counterflow pnenomenon is disadvantageous and involves an essential drawback.